This invention relates to a color reproduction system to effect automatically high speed color processing and the production of color copies and, more specifically, to a system for the automatic exposure of moving documents to be copied, and the superposition of sequential images of the documents on a final support medium.
In recent years there has been a number of attempts to use electrostatic printing for the production of faithful color renditions of multi-colored originals. One such system, described in U.S. Pat. No. 3,724,943 to Draugelis et al., uses an endless panchromatic photoconductor belt which is moved in an endless path having at least two flat runs. One of the flat runs is utilized at an exposure station at which a stationary color original is exposed by means of flash, full-frame exposure. The original being reproduced is subject to a series of successive, multiple exposures, and a light filter device, having a separation color filter for each color desired to be reproduced, is utilized and arranged to present one filter during each of the exposures so that the original is exposed once for each color rendition. Another flat run is utilized at a development station for developing the resultant electrostatic latent image. At this station there is positioned a plurality of in-line developing devices each of which is adapted to develop an image with a corresponding subtractive color material.
A programming arrangement is utilized along with the plurality of color filters for exposing the stationary original a plurality of times, once for each color, wherein successive images, each of a different color, are produced and wherein the plurality of latent images are presented in succession to the developing station. The programmer also controls the activation of each of the developing devices for respectively developing the latent images by a developing device. The programmer also includes means for controlling the disposition of a sheet of paper at a transfer station so that the sheet is brought into transfer relationship with the developed images in a recirculating basis; that is, one application of the sheet of paper into the photoconductive belt for each different color image to be transferred.
A problem posed by a system in which one developed separation color image is to be superimposed upon another, is ensuring that the developed images are placed in perfect registration upon each other at the transfer station. In the above-mentioned U.S. Pat. No. 3,724,943, use is made of a bias transfer roll, supporting a sheet material, which is successively moved into and out of contact with the photoconductor belt bearing the developed separation color images, and which is rotated an entire cycle between superposition of one separation image on another on the sheet material.
The system disclosed in U.S. Pat. No. 3,724,943 provides proper registration or superposition of the developed separation color images at the transfer station because a stationary original is used; that is an original which is fixed on a platen during exposure. This means that with a plurality of synchronized flash exposures of the original, the separation distance between the latent color images on the photoconductor is kept constant and placement of the latent images on a particular area of the belt is exactly provided. Thus, the latent images are placed on the photoconductor at a position with respect to the bias transfer roll such that the developed images will always be superimposed on the roll. As more fully described in the above-mentioned U.S. patent, an electro-mechanical programming or synchronization system is used, whereby the timing of the flash exposure is synchronized with movement of the bias transfer roll and with movement of the photoconductor belt.
In recent years, interest has been shown in an electrostatic color reproduction system in which it is required to copy a color document having its color information stored on a plurality of color separation masters. Each of these color separation masters include a black and white image having information as to the particular color content of the color document. For example, one master may be a black and white transparent film having only blue color information of the color document, a second film may be a black and white transparency having only red color information of the document, and a third film may be a black and white transparency having only green color information. The process of making such color separation masters is well-known.
To make a color reproduction, the color separation masters are spaced apart on a movable support and moved past the exposure station. As each separation master enters the exposure station, the image is exposed by means of a flash, full frame exposure and a latent image is placed on the moving photoconductor belt.
As may be seen from the above with respect to a system using a stationary original, it is important that the latent images be placed precisely on the photoconductor with respect to a particular circumferential position of the transfer roll occur to assure accurate registration of the separate color images on a transfer sheet material. However, with moving images to be copied, the correct placement of the latent images on the moving photoconductor, such that the separation distances are constant and the latent images are properly located on the photoconductor, is difficult to attain. For example, if the electro-mechanical system of U.S. Pat. No. 3,724,943 were used with moving color separation masters, flash exposure would always occur at a predetermined time in relation to movement of the bias transfer roll and photoconductor belt; however, whereas the first and second color separation masters may be properly located at the exposure station at the time of flash, the third color separation master might not be, whereby the separation distance between the latent images on the belt of the second and third color separation masters is different from the separation distance between the first and second latent images corresponding to the first and second color separation masters. Consequently, at the transfer station, the developed second latent image would be superimposed on the first developed image, but the third developed image would not be.
There are several reasons why the color separation images might not be properly positioned in the exposure station at the time of flash. Each of the color separation masters may be placed in a frame on a movable conveyor, and as the latter moves the masters may slide slightly within the frame. The masters cannot be very tightly fixed in the frames; otherwise, it would be difficult and time consuming for an operator to load the masters to make copies. Also, during movement of the conveyor for the masters, there may be some minute speed changes, resulting in delaying or advancing the placement of the masters in the proper location of the exposure station with respect to the time of flash. Furthermore, there might be slight changes in the speed of the transfer roll as transfer of the separation images occurs, which speed changes in the above-mentioned patent would result in variation of the timing of the flash. Misregistration of the color separation masters may be slight; however, this is sufficiently substantial to result in undesireable degradation of the color copy.